French patents FR 2 660 705 and FR 2 667 908 in the name of Regie Nationale des Usines Renault relate to servo-controlling the force of hydraulic actuators in a car suspension.
The devices described constitute active suspensions obtained by applying control theory optimized by minimizing quadratic criteria.
The term "active suspension" is used for suspensions that make use of energy generation, which energy may be hydraulic, pneumatic, or electrical, and with high levels of power being involved.
In addition, a suspension is set to be "semi-active" when it possesses variable characteristics that are controlled but without significant power being supplied.
A control relationship that takes account of measurements performed by sensors on a vehicle and representing movements of the vehicle make it possible to calculate the forces that the suspensions need to apply at any instant in order to minimize the desired criterion.
For example, the criterion may be seeking to reduce vibration or to limit vibration amplitude.
The forces calculated in this way constitute reference forces that the members of the suspension ought to follow. Such members then act as servo-controlled force generators with a force negative feedback loop.
Under ideal circumstances, each force servo-control loop corresponding to each of the suspension members of the vehicle is implemented by a hydraulic actuator powered at high pressure and provided with a force sensor and with a servo-valve.
Such a solution is described, for example, in French patents FR 2 660 705 and FR 2 667 907.
From patent FR 2 660 705 it can be seen that the essential problem lies in the difficulty of achieving force servo-control with an actuator when the actuator is itself subjected to large amplitude movements at relatively low frequency, as is the case in the context of a car suspension.
In other words, this difficulty gives rise to the car suspension having impedance that is highly variable.
Patent FR 2 660 705 describes a corrector for correcting the control current of the servo-valve on the basis of the measured relative speed between the two ends of the actuator.
The device described in patent FR 2 660 705 thus makes use of an inverse model of the servo-valve integrated in a positive correction loop which is superposed on the main feedback loop for force servo-control.
Another characteristic of the prior art device is that the inverse servo-valve model used also takes account of the force implemented, as measured, in addition to the relative speed, as measured.
The prior art device constitutes a suspension that is active in the sense that the device includes a high pressure hydraulic generator system.
An advantage of the semi-active damper of the invention is that it does not include hydraulic generation, because the device of the invention constitutes a device that is semi-active, in that it produces a damping relationship that is varied under control.
Another limitation of conventional damper devices is that the damping relationship is unique in character and is "frozen" by construction.